1. Field of the Invention
This invention relates to catalytic cracking of petroleum fractions. More particularly, this invention relates to an improved FCC process for converting petroleum fractions into valuable hydrocarbon products and coke.
2. Description of the Prior Art
Conversion of various petroleum fractions to more valuable products in catalytic reactors is well known in the art. The petroleum industry has found the use of a fluid bed catalytic cracking reactor (hereinafter FCC reactor) particularly advantageous for that purpose. An FCC reactor typically comprises a thermally balanced assembly of apparatus comprising the reactor vessel containing a mixture of regenerated catalyst and the feed and a regenerator vessel wherein spent catalyst is regenerated. The feed is converted in the reactor vessel over the catalyst, and coke simultaneously forms on the catalyst, thereby deactivating the same. The deactivated (spent) catalyst is removed from the reactor vessel and conducted to the regenerator vessel, wherein coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. The reactor-regenerator assembly must be maintained in steady state heat balance, so that the heat generated by burning the coke provides sufficient thermal energy for catalytic cracking in the reactor vessel. The steady state heat balance is usually achieved and maintained in FCC reactors by controlling the rate of flow of the regenerated catalsyt from the regenerator to the reactor by means of an adjustable slide valve in the regenerator-to-reactor conduit.
The product stream of the catalytic cracker is usually fractionated into a series of products, including: gas, normally conducted to gas treatment plant; gasoline; light cycle gas oil; and heavy cycle gas oil. A portion of the heavy cycle gas oil is usually recycled into the reactor vessel and mixed with fresh feed. The bottom effluent of the fractionator is conventionally subjected to settling and the solid-rich portion of the settled product is also recycled to the reactor vessel in admixture with the heavy cycle gas oil and feed.
In a modern version of FCC reactor, the regenerated catalyst is introduced into the base of a riser reactor column in the reactor vessel. A primary purpose of the riser reactor is to crack the petroleum feed. The regenerated hot catalyst is admixed in the bottom of the riser reactor with a stream of fresh feed and recycled petroleum fractions, and the mixture is forced upwardly though the riser reactor. During the upward passage of the catalyst and of the petroleum fractions, the petroleum is cracked, and coke is simultaneously deposited on the catalyst. The coked catalyst and the cracked petroleum components are passed upwardly out of the riser and through a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked petroleum fraction is conducted to product separation, while the coked catalyst, after steam stripping, passes into the regenerator vessel and is regenerated therein, as discussed above. Most of the cracking reactions in such modern FCC units take place in the riser reactor. Accordingly, the remainder of the reactor vessel is used primarily to separate entrained catalyst particles from the petroleum fractions.
Further details of FCC processes can be found in: U.S. Pat. Nos. 2,383,636 (Wurth); 2,689,210 (Leffer); 3,338,821 (Moyer et al); 3,812,029 (Snyder, Jr.); 4,093,537 (Gross et al); 4,118,337 (Gross et al); 4,118,338 (Gross et al), and, 4,218,306 (Gross et al), as well as in Venuto et al, Fluid Catalytic Cracking With Zeolite Catalysts, Marcel Dekher, Inc. (1979). The entire contents of all of the above patents and publications are incorporated herein by reference.
Performance characteristics of FCC reactors can be measured by a number of factors, e.g., percent conversion (usually in volume percent) of feed to all of the products of the FCC reactor, such as gasoline, coke and gas; selectivity which is a measure (also usually in percent volume) of the conversion of feed to gasoline grade products; and octane number of product gasoline. The production of higher octane gasoline is becoming increasingly important in modern refining. However, higher FCC gasoline octanes are usually difficult to obtain in conventional FCC installations without a corresponding loss in gasoline yield. The loss of gasoline yield cannot be tolerated, especially in view of relatively low profit margins realized by gasoline refiners under present economic conditions.
The use of steam, or other diluent vapors, for catalyst fluidization, unit start-up, residence time reduction and feed/catalyst mixing has been known in the FCC art. For example, James, U.S. Pat. Nos. 3,946,876 and 4,026,789, uses steam to help disperse the hydrocarbon material in the reactor vessel of an FCC plant. The steam also lowers the partial pressure of the hydrocarbon vapors to below about 20 pounds per square inch (e.g., column 4, lines 48-52 of the '789 patent). Several other patents also disclose the use of steam in the reactor to help fluidize the catalyst, aid in unit start-up, reduce residence time of the feed in the reactor and improve mixing of feed with the catalyst (e.g., Snyder, Jr., 3,812,029; Pappas, 3,074,878; Cartmell, 3,785,782; Payton, 3,042,196; and, Annesser, 3,243,265). However, there is no recognition in any of these patents that the addition of a certain, narrowly-defined amount of steam to the hydrocarbon feed in an FCC reactor, or the reduction of oil partial pressure in the reactor to within certain defined limits, may result in unexpected advantages achieved by this invention.
Accordingly, it is a primary object of the present invention to provide an improved process for producing gasoline having increased octane without the decrease of gasoline yield.
It is an additional object of this invention to provide a process for producing gasoline having increased octane by decreasing gaseous oil partial pressure in the FCC reactor riser.
These and other objects of this invention will become apparent to those skilled in the art from the study of the specification and appended claims.